Filters, MMed Physics, Lesson 6

Lesson 6: Filters, MMed Physics, Lesson 6

Filtration the process of shaping the x-ray beam to increase the ratio of photons useful for imaging to those photons that increase patient dose or decrease image contrast.

1)      Diagnostic x-ray beams are composed of photons that have a whole spectrum of energies; i.e polychromatic

2)      Their mean energies are from 1/3 to ½ of their peak energies, so many photons fall into the low energy range

3)      As polychromatic radiation pass through a patient most of the low energy photons are absorbed in the first few centimetres of tissue, only higher energy photons penetrate through the patient to form the radiographic image.

4)      The patient’s radiation dose is dependent on the number of absorbed photons, the first few centimetres of tissue receive much more radiation than the rest of the patient.

5)      This tissue can be protected by absorbing the lower energy photons from the beam before they reach the patient by interposing a filter between the x-ray tube and the patient. Reduction of skin exposures

6)      Filters are usually sheets of metal, placed in the path of the x-ray beam near the x-ray tube housing to absorb low energy radiation before it reaches the patient. Their main function in diagnostic radiology is to reduce the patient radiation dose. Heavy metal filters are also used to improve image contrast.

7)      X-ray beam is filtered by absorbers at 3 different levels:

a)      The x-ray tube and its housing (inherent filtration)

b)      Sheets of metal placed in the path of the x-ray beam (added filtration)

c)       The patient

8)      Inherent Filtration:

a)      Filtration that results from the absorption of x rays as they pass through the x ray tube and its housing is called inherent filtration. Materials responsible:

i)        glass envelope enclosing the anode and cathode,

ii)       the insulating oil surrounding the tube, and

iii)     the window in the tube housing.

b)      Measured in aluminium equivalents (0.5 to 1.0 mm aluminium)

c)       Beryllium window: Unfiltered beam is required in mammography, as increasing filtration increases the mean energy of a beam and decreases contrast. Beryllium window is used to produce an unfiltered beam.

9)      Added filtration

a)      Added filtration results from absorbers placed in the path of the x-ray beam. Ideally to absorb all the low energy photons and transmit all the high energy photons.

i)        Attenuation is most intense when the photoelectric effect predominates and diminishes when the Compton reactions predominate.

ii)      Aluminium (z=13) or a compound filter of Aluminium and Copper (z=29). The copper part faces the tube and the aluminium absorbs the characteristic radiation of from the copper, and faces the patient.

b)      Filter thickness

i)        Excessive filtration does not effect the quality of the beam but definitely diminishes the intensity of the beam.  This would lengthen the exposure time required and increase the likelihood of patient movement during examination.

ii)       Filtration reduces the total number of photons in the x-ray beam (area under the curve) and selectively removes a large number of low energy photons. The overall effect is to increase the mean energy of the x-ray beam.

iii)     National Council on Radiation Protection and Measurements has recommended the total filtration (both inherent and added filtration)

Operating kVp	Total Filtration
Below 50 kVp
50 to 70 kVp
Above 70 kVp

c)       Effect of Filters on Patient Exposure

i)        Adequate filtration can reduce patient exposure upto 80%

d)      Effect on Exposure factors

i)        Reduction in intensity of x-ray beam: major disadvantage of filters. Compensating for the loss of energy by increasing the exposure factor (mAs).

ii)      The total number of photons reaching the patient is still less than an unfiltered beam!

e)      Wedge Filters used to obtain films of more uniform density when part being examined diminishes greatly in thickness from one side of the field to the other. Figure 6.2. Used in lower limb angiography, when one images from the lower abdomen to the ankles.

10)   Heavy Metal Filters (K-edge filters)

a)      K-absorption edge of elements with atomic numbers greater than 60,

b)      offers advantages when imaging with barium and iodine

c)       Purpose of heavy metal filters is to produce an x-ray beam that has a high number of photons in the specific energy range that will be most useful in diagnostic imaging.

i)        Energy selective filters for diagnostic radiology Figure 6.3.

(1)    2mm Aluminium filter transmits a broad spectrum of bremsstrahlung.

(2)    Gadolinium 0.25mm has increasing numbers of photons range between 25 to 50.2 kVp. Above 50.2 kVp the mass attenuation coefficient increases dramatically and the number of protons transmitted is significantly diminished.

(a)    Reduction in low energy photons will reduce the patients absorbed dose.

(b)   Image contrast will be improved by reduction in higher energy photons (i.e. more photoelectric and less Compton attenuation).

(i)      Paediatric application: Improved contrast is maximal for thin body parts.  

(c)    Increase filtration increases tube loading (more mAs)

d)      Holminium filter (k-edge attenuation decreases from 33 to its K-edge of 55.6keV) used with Iodine (K-edge 33.17) and Barium Contrast (k-edge 37.45 keV). This allows a transmission window in the 33 to 55 keV range. This window allows for improved contrast.

11)   Molybdenum Filters:

a)      Mammography a molybdenum target x-ray tube to take advantage of the 17.5 keV K-alpha and the 19.6 keV K-beta characteristic radiation of molybdenum.

b)      Operated at 30 to 40 keV, produces significant Bremsstrauling with energies higher than 20 keV. Higher energy radiation will reduce contrast in breast tissue.

c)       To reduce the amount of radiation above 20 keV a 0.03 mm molybdenum filter is used.